In the underground coal gasification process, at least one injection borehole is drilled into the coal seam through which the combustion-supporting gas is supplied and a distance away is at least one production borehole where the combustion product gases are removed. During the gasification process, a cavity is formed in the coal seam as it is consumed. Frequently there is an aquifer above the coal. This aquifer is separated from the coal by a layer of shale or shaly sand which may not be adequate to keep undesired water from entering the cavity in excessive quantities.
In hydraulic borehole mining, a borehole is drilled into the formation to be mined. The mineral formation may contain coal, oil sand, uranium ore, phosphate rock, or some other substance. The borehole mining tool is inserted through this borehole where a water jet cuts the material into a slurry which is pumped back up through sections of the borehole mining tool via a jet pump. Thus a mined cavity is formed. Excessive water influx into this cavity from above can overload the jet pump and cause the water level to rise above the level of the water jet which will interfere with the cutting action of the jet.